Diesel fuel, also referred to as gas-oil, hereinafter, without distinction, simply, “diesel”, is a liquid fuel, substantially composed of saturated hydrocarbons (primarily paraffins including n- or iso-paraffins, and cycloparaffins) and aromatic hydrocarbons, which are used in Diesel engines or heating equipment. Diesel obtained by petroleum refining is called petrodiesel. In this context, diesel fuel is the fraction or cut of crude oil distilling between 200° C. and 370° C. at atmospheric pressure. There are other definitions for diesel fuel derived from other carbon sources, such as biodiesel, obtained from vegetable oils.
Diesel is mainly used in compression ignition engines, which operate according to the Diesel Cycle. One of the main quality standards of the diesel fuel is the “cetane number”, which measures the desirable autoignition tendency in such engines. Unlike the octane number for gasoline, the cetane number is not closely related to engine efficiency, but it will measure the emission generation tendency, particularly of hydrocarbons, carbon monoxide and particles, during engine operation. According to the cetane number scale, the autoignition response of methyl-naphtalene (aromatic hydrocarbon) is assigned a score of 0, and that of hexadecane (paraffinic hydrocarbon) is assigned a score of 100. A high content of aromatics in a diesel fuel will have an adverse impact on its cetane number.
In a traditional industrial petroleum refining process, most of the diesel produced is derived from the atmospheric distillation of crude oil. However, some part, currently near 30% of the diesel produced by refineries, is obtained by fluidized catalytic cracking (FCC). This process is mainly fed by a mixture of cuts referred to as atmospheric heavy gas oil (HGO), heavy coker gas oil (HCGO) and heavy vacuum gas oil (VGO), derived from previous processes. The main objective of FCC is obtaining high quality gasolines/naphthas, due to the high content of aromatic hydrocarbons, which have a positive impact on the octane number. Nevertheless, this characteristic also leads to a low cetane number and lower quality of the diesel obtained in this process. This is due to the fact that aromatic polycyclic compounds generate small fractions of coke and gas or remain without reaction in the presence of the acid catalytic in the FCC process and leave the unit as Light Cycle Oil (LCO) and Heavy Cycle Oil (HCO). This also limits the overall conversion of the process.
Another source of diesel in a traditional refinery is hydrocracking (HC), wherein catalytic cracking is associated to hydrogenation reactions. By means of this process, aromatic polycyclic compounds can lead to low-molecular-weight cyclic compounds within the gasoline and diesel boiling range.
As a result of this, both FCC and HC are processes that take place jointly in many refineries, in different combinations and receiving different feedstocks, depending on the specific objectives of the refinery. Several patent documents are known to mention processes for obtaining light hydrocarbons, such as naphtha and diesel, simultaneously applying HC and FCC processes, such as patent applications EP 436253 A1, JP 08-218082, EP 2154225 A1, WO 2009/085696 and US 2010/065475 A1, and patents U.S. Pat. No. 4,192,734, GB 1001449 A, GB 141306 A and U.S. Pat. No. 7,074,321 B1.
Such documents consider the simultaneous use of HC and FCC for treating feedstocks from several sources, in which each process has its own fractionation unit.
Patent application EP 436253 A1 refers to arrangement of HC and FCC processes in parallel, including feedstocks from different sources, in which each process has its own fractioning stage.
Patent application JP 08-218082 refers to obtaining a high quality diesel by mixing streams derived from HC and FCC processes.
Patent application EP 2154225 A1 refers to an integrated process for the conversion of heavy hydrocarbons to a light distillate and/or mid-distillate, combining a HC, a FCC and a hydroconversion process.
Patent application WO 2009/085696 refers to a process wherein HC and FCC processes are serially combined.
Patent application US 2010/065475 A1 refers to a FCC process, with pre-treatment by hydrocracking, for feedstocks derived from petroleum and biological sources. U.S. Pat. No. 4,192,734 refers to the production of high quality diesel from a stream resulting from the vacuum distillation of the atmospheric distillation residue, including the possibility to use HC and FCC in parallel.
Patent GB 1001449 A refers to a hydrocarbon conversion process combining HC and FCC with different feedstocks, but comprising individual fractioning in each process.
Patent GB 141306 A refers to obtaining high-octane gasoline by converting paraffinic naphtha by HC and FCC processes.
U.S. Pat. No. 7,074,321 B1 refers to the use of FCC for converting the unconverted fraction of the HC process.
Most of such patents are mainly intended to improve gasoline yield and quality, being the products derived from petroleum refining with highest commercial value. Based on current international market trends and ultra low sulfur requirements, there is an increasing need for the development of processes aimed at improving the yield and quality of the diesel obtained from refining, in particular, the diesel produced by FCC processes.